Development of imaging techniques of tissue characterization by means of time domain microwave computed tomography

1.研究结论：研究了利用时间域测量技术进行微波成像的可行性。在这项研究中，使用了时域测量技术来区分在两个天线之间的直线路径上传输的波分量和散射或反射波分量。这是一种类似于CP-MCT的技术，利用线性调频雷达的频域测量技术来区分两个天线之间的直线路径。研制了由矢量网络分析仪和微波扫描器组成的TD-MCT实验系统，在0.8 GHz到4.2 GHz的频率范围内测量了S21。两个天线之间的传输时间可以是时域的，信号是由将金属板放在天线孔径前面所产生的反射波决定的。因此，具有对应于…之间的直线路径的特定时间间隔的信号可以从观测信号中去掉两个以上的天线，以在频域中形成投影数据。在从1 GHz到4 GHz的频率范围内，以每500 MHz重复该图像重建过程。观察到的主要差异是衍射波和透射波之间的干扰在信噪比和不可避免的伪影数量方面的差异。在1 GHz带宽的图像重建中，利用1 GHz到2 GHz的频率分量重建的图像质量最好。结果表明，设计在1 GHz~2 GHz的CP-MCT是合理的。TD-MCT的估计空间分辨率与CP-MCT的空间分辨率相同。时间域微波CT的实际意义：一般来说，任何一台微波CT扫描仪都必须在生理盐水中浸泡天线和生物物体进行测量，以实现天线与生物物体之间的阻抗匹配。在微波成像频率范围内，生理盐水溶液的介电常数与生物组织的介电常数相近。通过尽可能地增加信号的带宽，最有效地获得了时域测量的优势。另一方面，由于传输损耗随着频率的增加而急剧增加，因此很难提高层析测量的频率。也很难实现带宽很宽的天线，例如在盐水中超过3 GHz。CP-MCT和TD-MCT在频域提取直路传输分量，TD-MCT在时间域提取直线传输分量。然而，CP-MCT可以通过测量低于1 kHz或更小的甚低频信号来实现这一点。与时间域中的测量过程相比，这是一项简单的任务。认为当频谱信息得到有效利用时，TD-MCT的优势就会显现出来。较少

1. Conclusions of the study : Feasibility of microwave imaging using measurement techniques in time domain has been investigated. Time domains measurement techniques are used in this study for discriminating the wave components that transmitted on the straight path between two antennas from the scattered- or reflected wave components. This is a similar technique to CP-MCT in which the straight path between two antennas is discriminated by use of the techniques in frequency domain measurement of a chirp radar.The developed experimental system of Time Domain Microwave Computed Tomography (TD-MCT) that is consisted of a vector network analyzer and microwave scanner and measures S21 in the frequency range from 0.8 GHz to 4.2 GHz. Transmission time between two antennas can be time domain signal is determined by the reflected waves made by placing a metal plate in front of the antenna aperture. Accordingly, the signal which has a specific time interval corresponding to the straight path betw … More een two antennas can be excised from the observed signal to make projection data in frequency domain. This image reconstruction procedure is repeated at every 500 MHz in the frequency range from 1 GHz to 4 GHz. The major differences were observed in the signal to noise ratio and amount of unavoidable artifacts caused by interference among the diffracted- and transmitted-waves. In image reconstruction with 1 GHz bandwidth, the image reconstructed by use of the frequency components from 1 GHz to 2 GHz exhibits the most excellent quality. It is concluded that the CP-MCT that was designed at 1 GHz to 2 GHz has been properly designed. The estimated spatial resolution of TD-MCT shows the same value with that of CP-MCT.2. Practical significance of time domain microwave computed tomography : Generally speaking, any scanners of microwave computed tomography have to make measurement in bolus saline solution by immersing the antennas and biological object for impedance matching between them. Saline solution shows similar permittivity values with biological tissues in the frequency range of microwave imaging. The advantages of time domain measurement are derived most effectively by increasing the bandwidth of the signal as much as possible. On the other hand, it is hard to increase the frequency of tomographic measurement because transmission loss is increased abruptly when frequency increases. It is also hard to realize the antenna with a very wide bandwidth, for example more than 3 GHz in saline solution. CP-MCT extracts straight path transmission components from transmitted waves in frequency domain and TD-MCT does the same thing in time domain. However, CP-MCT can do it by measuring very low frequency signal which is lower than 1 kHz or less. This is an easy task as compared to the measurement procedure in time domain. It is considered that the advantages of TD-MCT will be found when spectral information can be used effectively. Less